313 lines
8.8 KiB
Rust
313 lines
8.8 KiB
Rust
//! Standard library macros
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//!
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//! This module contains a set of macros which are exported from the standard
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//! library. Each macro is available for use when linking against the standard
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//! library.
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#[doc(include = "../../core/src/macros/panic.md")]
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#[macro_export]
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#[stable(feature = "rust1", since = "1.0.0")]
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#[allow_internal_unstable(libstd_sys_internals)]
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macro_rules! panic {
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() => ({ $crate::panic!("explicit panic") });
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($msg:expr) => ({ $crate::rt::begin_panic($msg) });
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($msg:expr,) => ({ $crate::panic!($msg) });
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($fmt:expr, $($arg:tt)+) => ({
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$crate::rt::begin_panic_fmt(&$crate::format_args!($fmt, $($arg)+))
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});
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}
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/// Prints to the standard output.
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///
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/// Equivalent to the [`println!`] macro except that a newline is not printed at
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/// the end of the message.
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///
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/// Note that stdout is frequently line-buffered by default so it may be
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/// necessary to use [`io::stdout().flush()`][flush] to ensure the output is emitted
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/// immediately.
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///
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/// Use `print!` only for the primary output of your program. Use
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/// [`eprint!`] instead to print error and progress messages.
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///
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/// [flush]: crate::io::Write::flush
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///
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/// # Panics
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///
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/// Panics if writing to `io::stdout()` fails.
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///
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/// # Examples
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///
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/// ```
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/// use std::io::{self, Write};
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///
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/// print!("this ");
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/// print!("will ");
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/// print!("be ");
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/// print!("on ");
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/// print!("the ");
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/// print!("same ");
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/// print!("line ");
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///
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/// io::stdout().flush().unwrap();
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///
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/// print!("this string has a newline, why not choose println! instead?\n");
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///
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/// io::stdout().flush().unwrap();
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/// ```
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#[macro_export]
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#[stable(feature = "rust1", since = "1.0.0")]
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#[allow_internal_unstable(print_internals)]
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macro_rules! print {
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($($arg:tt)*) => ($crate::io::_print($crate::format_args!($($arg)*)));
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}
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/// Prints to the standard output, with a newline.
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///
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/// On all platforms, the newline is the LINE FEED character (`\n`/`U+000A`) alone
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/// (no additional CARRIAGE RETURN (`\r`/`U+000D`)).
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///
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/// Use the [`format!`] syntax to write data to the standard output.
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/// See [`std::fmt`] for more information.
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///
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/// Use `println!` only for the primary output of your program. Use
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/// [`eprintln!`] instead to print error and progress messages.
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///
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/// [`std::fmt`]: crate::fmt
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///
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/// # Panics
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///
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/// Panics if writing to [`io::stdout`] fails.
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///
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/// [`io::stdout`]: crate::io::stdout
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///
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/// # Examples
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///
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/// ```
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/// println!(); // prints just a newline
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/// println!("hello there!");
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/// println!("format {} arguments", "some");
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/// ```
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#[macro_export]
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#[stable(feature = "rust1", since = "1.0.0")]
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#[allow_internal_unstable(print_internals, format_args_nl)]
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macro_rules! println {
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() => ($crate::print!("\n"));
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($($arg:tt)*) => ({
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$crate::io::_print($crate::format_args_nl!($($arg)*));
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})
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}
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/// Prints to the standard error.
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///
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/// Equivalent to the [`print!`] macro, except that output goes to
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/// [`io::stderr`] instead of [`io::stdout`]. See [`print!`] for
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/// example usage.
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///
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/// Use `eprint!` only for error and progress messages. Use `print!`
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/// instead for the primary output of your program.
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///
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/// [`io::stderr`]: crate::io::stderr
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/// [`io::stdout`]: crate::io::stdout
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///
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/// # Panics
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///
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/// Panics if writing to `io::stderr` fails.
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///
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/// # Examples
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///
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/// ```
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/// eprint!("Error: Could not complete task");
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/// ```
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#[macro_export]
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#[stable(feature = "eprint", since = "1.19.0")]
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#[allow_internal_unstable(print_internals)]
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macro_rules! eprint {
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($($arg:tt)*) => ($crate::io::_eprint($crate::format_args!($($arg)*)));
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}
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/// Prints to the standard error, with a newline.
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///
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/// Equivalent to the [`println!`] macro, except that output goes to
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/// [`io::stderr`] instead of [`io::stdout`]. See [`println!`] for
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/// example usage.
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///
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/// Use `eprintln!` only for error and progress messages. Use `println!`
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/// instead for the primary output of your program.
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///
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/// [`io::stderr`]: crate::io::stderr
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/// [`io::stdout`]: crate::io::stdout
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///
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/// # Panics
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///
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/// Panics if writing to `io::stderr` fails.
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///
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/// # Examples
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///
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/// ```
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/// eprintln!("Error: Could not complete task");
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/// ```
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#[macro_export]
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#[stable(feature = "eprint", since = "1.19.0")]
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#[allow_internal_unstable(print_internals, format_args_nl)]
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macro_rules! eprintln {
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() => ($crate::eprint!("\n"));
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($($arg:tt)*) => ({
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$crate::io::_eprint($crate::format_args_nl!($($arg)*));
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})
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}
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/// Prints and returns the value of a given expression for quick and dirty
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/// debugging.
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///
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/// An example:
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///
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/// ```rust
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/// let a = 2;
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/// let b = dbg!(a * 2) + 1;
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/// // ^-- prints: [src/main.rs:2] a * 2 = 4
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/// assert_eq!(b, 5);
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/// ```
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///
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/// The macro works by using the `Debug` implementation of the type of
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/// the given expression to print the value to [stderr] along with the
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/// source location of the macro invocation as well as the source code
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/// of the expression.
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///
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/// Invoking the macro on an expression moves and takes ownership of it
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/// before returning the evaluated expression unchanged. If the type
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/// of the expression does not implement `Copy` and you don't want
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/// to give up ownership, you can instead borrow with `dbg!(&expr)`
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/// for some expression `expr`.
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///
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/// The `dbg!` macro works exactly the same in release builds.
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/// This is useful when debugging issues that only occur in release
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/// builds or when debugging in release mode is significantly faster.
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///
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/// Note that the macro is intended as a debugging tool and therefore you
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/// should avoid having uses of it in version control for long periods.
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/// Use cases involving debug output that should be added to version control
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/// are better served by macros such as [`debug!`] from the [`log`] crate.
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///
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/// # Stability
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///
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/// The exact output printed by this macro should not be relied upon
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/// and is subject to future changes.
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///
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/// # Panics
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///
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/// Panics if writing to `io::stderr` fails.
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///
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/// # Further examples
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///
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/// With a method call:
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///
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/// ```rust
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/// fn foo(n: usize) {
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/// if let Some(_) = dbg!(n.checked_sub(4)) {
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/// // ...
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/// }
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/// }
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///
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/// foo(3)
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/// ```
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///
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/// This prints to [stderr]:
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///
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/// ```text,ignore
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/// [src/main.rs:4] n.checked_sub(4) = None
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/// ```
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///
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/// Naive factorial implementation:
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///
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/// ```rust
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/// fn factorial(n: u32) -> u32 {
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/// if dbg!(n <= 1) {
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/// dbg!(1)
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/// } else {
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/// dbg!(n * factorial(n - 1))
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/// }
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/// }
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///
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/// dbg!(factorial(4));
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/// ```
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///
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/// This prints to [stderr]:
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///
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/// ```text,ignore
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/// [src/main.rs:3] n <= 1 = false
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/// [src/main.rs:3] n <= 1 = false
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/// [src/main.rs:3] n <= 1 = false
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/// [src/main.rs:3] n <= 1 = true
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/// [src/main.rs:4] 1 = 1
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/// [src/main.rs:5] n * factorial(n - 1) = 2
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/// [src/main.rs:5] n * factorial(n - 1) = 6
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/// [src/main.rs:5] n * factorial(n - 1) = 24
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/// [src/main.rs:11] factorial(4) = 24
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/// ```
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///
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/// The `dbg!(..)` macro moves the input:
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///
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/// ```compile_fail
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/// /// A wrapper around `usize` which importantly is not Copyable.
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/// #[derive(Debug)]
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/// struct NoCopy(usize);
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///
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/// let a = NoCopy(42);
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/// let _ = dbg!(a); // <-- `a` is moved here.
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/// let _ = dbg!(a); // <-- `a` is moved again; error!
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/// ```
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///
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/// You can also use `dbg!()` without a value to just print the
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/// file and line whenever it's reached.
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///
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/// Finally, if you want to `dbg!(..)` multiple values, it will treat them as
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/// a tuple (and return it, too):
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///
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/// ```
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/// assert_eq!(dbg!(1usize, 2u32), (1, 2));
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/// ```
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///
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/// However, a single argument with a trailing comma will still not be treated
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/// as a tuple, following the convention of ignoring trailing commas in macro
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/// invocations. You can use a 1-tuple directly if you need one:
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///
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/// ```
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/// assert_eq!(1, dbg!(1u32,)); // trailing comma ignored
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/// assert_eq!((1,), dbg!((1u32,))); // 1-tuple
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/// ```
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///
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/// [stderr]: https://en.wikipedia.org/wiki/Standard_streams#Standard_error_(stderr)
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/// [`debug!`]: https://docs.rs/log/*/log/macro.debug.html
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/// [`log`]: https://crates.io/crates/log
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#[macro_export]
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#[stable(feature = "dbg_macro", since = "1.32.0")]
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macro_rules! dbg {
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() => {
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$crate::eprintln!("[{}:{}]", $crate::file!(), $crate::line!());
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};
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($val:expr) => {
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// Use of `match` here is intentional because it affects the lifetimes
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// of temporaries - https://stackoverflow.com/a/48732525/1063961
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match $val {
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tmp => {
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$crate::eprintln!("[{}:{}] {} = {:#?}",
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$crate::file!(), $crate::line!(), $crate::stringify!($val), &tmp);
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tmp
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}
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}
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};
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// Trailing comma with single argument is ignored
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($val:expr,) => { $crate::dbg!($val) };
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($($val:expr),+ $(,)?) => {
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($($crate::dbg!($val)),+,)
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};
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}
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#[cfg(test)]
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macro_rules! assert_approx_eq {
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($a:expr, $b:expr) => {{
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let (a, b) = (&$a, &$b);
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assert!((*a - *b).abs() < 1.0e-6, "{} is not approximately equal to {}", *a, *b);
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}};
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}
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